The extension of 193 nm optical lithography to numerical aperture (NA) values above 1.0, enabled by immersion optical projection systems, provides a means of achieving increased resolution for a printable minimum feature size, and therefore allows for further scaling of integrated circuits (IC) by the semiconductor industry.
Current state-of-the-art techniques in optical projection printing (such as 193 nm immersion lithography at NA=1.2) can resolve features beyond 50 nm half-pitch in photoresists with good linewidth control when planar, low reflectivity substrates are used. However, when photoresists are exposed on reflective substrates in the presence of underlying surface topography, critical dimension (CD) control problems are exacerbated under high NA imaging conditions, and lead to the deterioration of the quality of the printed image.
Reflection of light from the substrate/resist interface produces variations in the light intensity and scattering in the resist during exposure, resulting in non-uniform photoresist linewidth upon development. Light can scatter from the interface into regions of the resist where exposure was not intended, resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region resulting in linewidth non-uniformity.
To eliminate the effects of chromatic aberration in exposure equipment lenses, monochromatic or quasi-monochromatic light is commonly used in resist projection techniques. Unfortunately, due to resist/substrate interface reflections, constructive and destructive interference is particularly significant when monochromatic or quasi-monochromatic light is used for photoresist exposure. In such cases the reflected light interferes with the incident light to form standing waves within the resist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed resist at the wave minima. If the resist thickness is non-uniform, the problem becomes more severe, resulting in variable linewidth control.
More specifically related to high NA optical imaging, photolithographic systems that utilize high NA lenses cause light to diffract at high angles. This deviation from normal incidence causes increased reflectance at the resist-air and resist-substrate interfaces, thus exacerbating the problem. Increased reflectance in turn causes an increase in both standing waves and CD swing.
In addition to the challenges posed by the use of high NA optical systems described above, additional difficulties arise in this field due to the fundamental loss in image contrast that occurs for the transverse magnetic (TM or p-) polarization state at large oblique angles (T. Brunner et al., Proceedings of SPIE Vol. 4691, p. 1, 2002). This loss in TM image contrast at high angles adds up to other sources of image contrast degradation such as blocking of diffraction orders at the pupil edge, defocus effects, image flare effects, stage vibrations, etc.
Linewidth control problems due to non-uniform reflectance also arise from substrate topography. Any image on the wafer will cause impinging light to scatter or reflect in various uncontrolled directions (reflective notching), affecting the uniformity of resist development. As the topography becomes more complex with efforts to design more complex circuits, the effects of reflected light become much more critical (H. Yoshino et al., Journal of Vacuum Science and Technology B, Vol. 15, p. 2601, 1997).
As a result of the optical effects at high NA and reflective notching described above, extending the resolution capability of 193 nm lithography requires reflectivity control over a wider range of angles.
A common method to address problems related to reflectivity control within imaging layers, is to apply an antireflective coating (ARC). A top ARC (TARC) deposited over the photoresist layer can significantly reduce the swing effect by reducing the reflectivity at the air-photoresist interface, However a TARC does not reduce the notching problem. Instead, a bottom ARC (BARC) formed beneath the photoresist layer is capable of eliminating both the swing and notching problems, and has emerged as the most effective reflectivity solution while interfering the least with the lithographic process.
Two types of BARC layers are commonly used by the semiconductor industry. Spin-on BARCs are typically organic materials applied as a liquid formulation to the semiconductor substrate from a spin-coating station (track). After the BARC film is formed, a high temperature bake (post-apply bake) is used to remove the casting solvent and to crosslink the polymer components, so as to form a BARC layer that is impervious to the casting solvent used in the photoresist formulation that is coated subsequently. In this case, the optical properties are defined by the chemical functionality of the polymer components present in the formulation.
Alternatively, BARCs deposited through radiation assisted techniques such as chemical vapor deposition (CVD), high density plasma, sputtering, ion beam or electron beam are typically inorganic or hybrid materials (e.g. silicon nitrides, silicon oxynitrides, hydrogenated silicon carboxynitrides, or combinations thereof) that are applied from a gas phase in a stand-alone deposition chamber, utilizing precursors capable of being volatilized, combined with gaseous co-reactants and converted to their corresponding hybrid or inorganic derivatives at high temperatures or assisted by plasma conditions. In this case, the chemical nature of the precursors and the reactant concentration ratios define the net chemical composition and the optical properties of the deposited BARC layer.
In any case, as the NA exceeds 1.0, a homogeneous single layer bottom antireflective coating (BARC) may not suffice in keeping substrate reflectivity below 1% at all incident angles, as indicated by Abdallah et al. (Proceedings of SPIE, Vol. 5753, p. 417, 2005). Instead, strategically structuring BARCs has been reported as the preferred approach to ameliorate the detrimental side effects of high-NA imaging and reflective notching when practicing high resolution lithography (K. Babich et al., Proceedings of SPIE, Vol. 5039, p. 152, 2003). Such strategy includes the use of discrete or continuous bottom antireflective multilayers with optical properties defined throughout the antireflective element(s) in such a way that the optical constants at the top of the BARC surface are approximately or identically equal to those of the photoresist at the exposure wavelength, to minimize reflection at the photoresist-BARC interface. The bottom section of the BARC is highly absorbing at the exposure wavelength, to minimize reflection from the ARC-substrate interface back into the photoresist. This idea has been accomplished by the use of either a multilayer BARC or a continuously graded BARC.
In the case of a multilayer BARC, two or more antireflective layers with distinct and properly selected refractive index (n) and absorption coefficient (k) are consecutively applied on the semiconductor substrate, thus forming an antireflective stack with enhanced optical properties with respect to a single layer BARC. The simplest case for a multilayer BARC, namely a dual-layer BARC, has been previously described as being effective at reducing unwanted reflectivity in semiconductor substrates, by using combinations of all-organic (Abdallah et al., Proceedings of SPIE Vol. 5753, p. 417, 2005), organic-inorganic (Ghandehari et al., U.S. Pat. No. 6,867,063) or all-inorganic materials (Linliu et al., U.S. Pat. No. 6,479,401).
Continuously graded BARC films with n and k values that can be tuned and varied throughout the depth of the antireflective layer can be generated using plasma-enhanced chemical vapor deposition (PECVD) methods, where the reactant feed ratios are continuously changed during the CVD BARC deposition process. Such is the case for the deposition of graded silicon oxycarbide (U.S. Pat. No. 6,297,521), graded silicon oxynitride (U.S. Pat. No. 6,379,014) or graded hydrogenated silicon carboxynitride (U.S. Pat. No. 6,514,667) BARC layers. Alternatively, a chemically uniform CVD-deposited BARC layer can be optically graded by chemically modifying the top surface with a plasma treatment (Applied Optics, Vol. 43, p. 2141, 2004).
The advantageous optical properties of structured BARCs such as those composed of a multilayered or graded antireflective film are met at the inevitable expense of added complexity to the lithographic process. A simple spin-on dual-layer BARC requires the use of two separate formulations and coating steps, which can increase the number of defects introduced on the substrate before the photoresist layer is applied, and represents a reduction in wafer throughput. Analogously, a graded CVD BARC necessitates a separate deposition chamber, which adds to the cost of the manufacturing process, and also represents a throughput reduction with respect to an all-track processing, due to the need to transport the wafers from the track to the CVD tool and back (“How AR Coatings Stack Up”, L. Peters; Semiconductor International, September 2005).